1. Field of the Invention
This invention relates to a light modulating device, especially a pixellated electro-optic light modulating device. In particular the invention relates to a liquid crystal device having a repeating pattern of differing properties that gives a pre-determined area for each region of a particular property with minimum error and without requirement for precise alignment or registration of the repeating pattern and addressing electrodes, and to a method of fabricating such a device.
2. Description of the Prior Art
Liquid crystal displays comprise electrode structures on one or more inner surfaces of the device to form a multitude of picture elements, or pixels. A common arrangement for the pixels is a rectangular array or matrix, since this is ideally suited for the display of graphical information. The pixels may be further sub-divided into separately electrically addressable areas, for example to provide greyscale and/or colour.
The internal surface of the device may include other layers over the electrode structure, including alignment layers, passive optical components (such as compensation films) and spacers (including wall structures). Patterning the structure of these layers within each pixel can give improved performance of the liquid crystal device. For example, varying the alignment direction that the alignment layer imparts to the director of a contacting liquid crystal within each pixel may lead to an improved viewing angle (see for example, V. G. Chigrinov (1999) “Liquid Crystal Device: Physics and Applications”, p64-65). Patterning passive retardation layers also gives improved optical characteristics, for example for transflective displays (e.g. B. M. I van der Zande, et al SID 03 Digest, pp 194-197). Spacer structures such as pillars can be deposited to provide and maintain accurate spacing between the substrates of the device. Here, the wall material does not contribute to the electro-optic nature of the pixel, and is advantageously positioned to be within the inter-pixel gap area of the device. Transflective displays may require that a certain area of each pixel has a cell gap suited for reflective mode operation, whilst the rest of the pixel has a different cell gap designed to give the best performance whilst operating in transmissive mode.
Patterning of the properties of a display device within each pixel is also important for bistable or multistable devices. Bistable displays are inherently digital in nature, i.e. either the pixel is in one state or the other. However for displaying images it is preferable to have a level of contrast or greyscale for the image. Indeed an essential part of producing colour displays is the achievement of sufficient greyscale. For example, achievement of 4096 colours requires three separately coloured sub-pixels each capable of 16 distinct transmission or reflection levels.
Various mechanisms for achieving greyscale are known. Full colour bistable ferroelectric liquid crystal displays are known (N. Itoh et al. “17” Video-rate Full colour FLCD”, Proc. 5th International Displays Workshops, Kobe, Japan, pp205-208 (1998). Here 256 greys were achieved using a combination of spatial dither and temporal dither.
Spatial dither uses spatial subdivision to latch varying amounts of the pixel into each bistable state. Temporal dither divides the frame into sub-divisions each of which can be used to display a different image. Temporal dither however requires fast operation and also requires constant update, reducing the usefulness of bistable displays as low power devices. A high level of spatial dither is costly, both in terms of the additional electronic drivers needed, and the reduced etching yield for the least significant (i.e. smallest) electrodes. Also, the gap between the pixels does not contribute to the electro-optic effect of the pixel, and so sub-dividing the pixels reduces the aperture ratio of the pixel, thereby reducing the maximum brightness and contrast ratio of the device.
Another approach is to generate greyscale through analogue levels. This is done using partial latching of the pixel. After blanking the pixel into one stable state an intermediate voltage level is applied. The applied voltage is insufficient to latch all of the pixel but nucleates domains of the opposite stable state and forms a random mixture of domains. Varying the applied signal can case the number and size of the domains to change leading to a continues change in the transmission or reflection of the pixel. This approach is often used for bistable cholesteric liquid crystal devices X-Y. Huang et al. “Gray scale of bistable reflective cholesteric displays”, Proc SID XXIX, LP. 1, pp810-813 (1998). However use of analogue levels in this way is dependent on the applied voltage, cell gap and temperature. Numerous variations that may occur across a panel, or from panel to panel need to be considered, including local alignment or temperature differences within the panel, transmission line losses associated with long thin electrodes, differences between drivers—either random or due to operating temperature—changes of cell gap associated with the flatness of the glass, or variation of the domain nucleation sites. Any of these variations will cause some change in the transmission or reflection from the pixel. This is shown in FIG. 1 where slight variations across a cell, such as ΔV lead to relatively large transmission errors ΔT. The total number of greys that can be achieved is related to the acceptable change in transmission caused by the variations, which is in turn related to the gradient of the latching characteristic. Attempts to widen the partial latch width to increase the number of analogue levels that may be achieved often results in an increased number of manufacturing steps.
U.S. Pat. No. 6,094,187 describes a ferroelectric liquid crystal device wherein greyscale is achieved by a combination of dither, either spatial or temporal, with the use of analogue levels. The pixel is divided into a number of bits which may be either spatial or temporal or both. At least two of the bits are addressed with more than two grey levels, i.e. more than just black and white transmission/reflection, and at least one bit is addressed with a lesser number of grey levels. This permits a high number of greys to be achieved.
Again however the analogue levels achieved will be susceptible to temperature variations and a large number of spatial or temporal bits require additional circuitry and faster addressing.
An alternative approach to providing greyscale into a bistable ferroelectric liquid crystal device is to sub-divide each pixel into to areas of differing response, for example, as described in Bonnett et al (1997) Proceedings of International Displays Research Conference, p L-46. For example, the electric field may be varied by sub-dividing the pixel into areas of different cell gap and/or use of passive dielectric layers disposed between the electrode and liquid crystal material.
Zenithal bistable devices (ZBDs) are described in Bryan-Brown et al. “Grating Aligned Bistable Nematic Device”, Proc SID XXVIII, 5.3, pp 37-40 (1997) and U.S. Pat. No. 6,249,332. These use a surface alignment layer to give two stable states of a nematic liquid crystal material having either high or low surface tilt. The grating may be manufactured using either standard photolithographic methods or by embossing into a conformable layer on one of the inner surfaces of the display. When used opposite a conventional rubbed alignment surface the device may be latched between Hybrid Aligned Nematic (HAN) and Twisted Nematic (TN) configurations. See FIG. 2. The device is latched between states using electrical pulses of sufficient impulse, τV, where τ is the pulse duration and V its amplitude. In practice a display is addressed a line at a time using bipolar strobe, Vs, and data, Vd, pulses applied to the row and column electrodes simultaneously. Bipolar pulses are required to prevent unwanted latching effects due to a net DC across the pixel. The line-address-time is then equal to two time slots. Latching occurs on the trailing pulse of the high voltage resultant |Vs+Vd|. The leading pulse acts to both DC balance the waveform and to pole the ionic field before the latching pulse. The pixel remains unchanged with the opposite sign of data by ensuring that the low voltage resultant |Vs −Vd| is below the latching threshold.
Black and white ZBD displays are described in E. L. Wood et al. “Zenithal bistable device (ZBD) suitable for portable applications”, Proceedings of SID, 2000, v31, 11.2, p124-127 (2000) that show good front of screen performance combined with ultra-low power and rugged image storage. A 5 μm cell gap is used with manufacturing tolerances closer to those of conventional twisted nematic (TN) displays rather then Supertwisted nematic (STN) displays. These high tolerances allow complex displays to be fabricated readily using plastic substrates.
Greyscale has previously been achieved in a ZBD device by use of regions having different latching properties. A pixel is sub-divided into various regions, each having a different latching property. The sub-divisions, which may be termed latching regions, are designed to give separately addressable areas using the using just one set of electrodes and drivers, each giving a discriminating operating window. Within this window the state of the pixel, and hence its transmission level, is insensitive to any variations of the latching threshold that may occur and may be termed ‘error-free’. Examples of multiple threshold techniques include varying the cell gap as shown in U.S. Pat. No. 4,712,877 or the applied field using electrode slits.
Alternatively the shape and alignment properties of the grating may be varied across a pixel, for example to give wide viewing angle and analogue greyscale Bryan-Brown et al. “Optimisation of the Zenithal Bistable Nematic Liquid Crystal Device” Proceedings of the 18th IDRC, Seoul, Korea, pp 1051-1053 (1998). For example each sub-pixel can be sub-divided into a number of areas with different latching thresholds. The fraction of the pixel that changes state, and hence its transmission, is then related to the applied electric signal.
Further when the shape and/or alignment properties of the grating are varied it is necessary to exactly align the variation in grating property with the addressing electrodes when the cell is fabricated. This can lead to complicated and relatively expensive alignment steps in manufacture. Any error in alignment will affect the proportion of each latching region in any pixel or sub-pixel with detrimental results on the display.
A common feature to each of these patterned devices is that the sub-division of each pixel must be uniform across the display. Each pixel is required to have two or more regions of differing property or properties (e.g. alignment direction, cell gap, electric field, bistable latching threshold, anchoring energy, optical retardation, dielectric constant) where the percentage of the pixel area formed by regions having the same properties is the same throughout all of the pixels in the device. Herein, the area is the same to an extent that differences in the sub-pixel areas that occur across the panel are not noticeable under normal viewing conditions. Hence, it is important to minimise the error of such sub-divisions across a panel. For this reason, prior art methods rely on precise techniques to ensure that the pattern associated with the varying property is aligned accurately with the electrode pattern. This has implications for the manufacture of such devices. Firstly, the alignment/registration of the repeating pattern of differing property to the underlying electrode structure requires costly mask-alignment equipment and reduces both yield and throughput of fabrication. Moreover, the repeating pattern must be changed wherever the layout of the electrodes varies. Hence, a different mask is required for each display format produced on the manufacturing line. Such issues are particularly important where the pattern is fabricated using an embossing approach. For example, a transfer layer with the repeating pattern may be used to impress areas of different dielectric thickness, or grating shape etc, and it is difficult to register the repeating pattern to the electrode structure accurately. If the transfer layer with the repeating pattern is provided on a roll (for a roll to roll process) then it is costly to stop the process to change the roll if the electrode format is changed mid-process. Moreover, the direction of the electrodes on the substrates may vary from panel to panel on each glass sheet (for instance, to ensure maximum usage of glass), which would not be possible without suitable registration of the transfer layer regions and the electrode structures.
Patterning of the layers used in electronic and display devices often relies on photolithographic steps in which the layer is exposed to radiation through an appropriate mask. Controlling the proportions (or indeed positions) of the patterns requires Mask Alignment equipment. A number of mask steps may be required to pattern complex structures adding to the expense of fabrication, and reducing both yield and throughput. Moreover, different electrode arrangements necessitate the mask to be changed appropriately.